1. Field of the Invention
This invention relates generally to the use of software simulation (which is intended to include firmware simulation) in testing of transmitters used in communications links, for example the testing of transmitters used in optical fiber communications link by software simulation of a reference channel (i.e., a reference optical fiber link) and/or a reference receiver.
2. Description of the Related Art
Optical fiber is widely used as a communications medium in high speed digital networks, including local area networks (LANs), storage area networks (SANs), and wide area networks (WANs). There has been a trend in optical networking towards ever-increasing data rates. While 100 Mbps was once considered extremely fast for enterprise networking, attention has recently shifted to 10 Gbps, 100 times faster. As used in this application, 10 Gigabit (abbreviated as 10G or 10 Gbps) systems are understood to include optical fiber communication systems that have data rates or line rates (i.e., bit rates including overhead) of approximately 10 Gigabits per second.
Regardless of the specific data rate, application or architecture, communications links (including optical fiber communications links) invariably include a transmitter, a channel and a receiver. In an optical fiber communications link, the transmitter typically converts the digital data to be sent to an optical form suitable for transmission over the channel (i.e., the optical fiber). The optical signal is transported from the transmitter to the receiver over the channel, possibly suffering channel impairments along the way, and the receiver then recovers the digital data from the received optical signal.
For example, a typical 10G optical fiber communications link 100 is shown in FIG. 1. The link 100 includes a transmitter 105 coupled through optical fiber 110 (the channel) to a receiver 120. A typical transmitter 105 may include a serializer, or parallel/serial converter (P/S) 106 for receiving 10G data from a data source on a plurality of parallel lines and providing serial data to a 10G laser driver 107. The driver 107 then drives a 10G laser source 108, which launches the optical waveform carrying the digital data on optical fiber 110.
On the receive side, a typical receiver 120 includes a 10G photodetector 111 for receiving and detecting data from the optical fiber 110. The detected data is typically processed through a 10G transimpedance amplifier 112, a 10G limiting amplifier 113, and a 10G clock and data recovery unit 114. The recovered data may then be placed on a parallel data interface through a serial/parallel converter (S/P) 115.
Standards play an important role in networking and communications. Since components in the network may come from different vendors, standards ensure that different components will interoperate with each other and that overall system performance metrics can be achieved even when components are sourced from different vendors. For example, standards for transmitters can be used to ensure that, when compliant transmitters are combined with a compliant channel and compliant receivers, the overall link will meet certain minimum performance levels. As a result, manufacturers of transmitters typically will want to test their transmitters for compliance with the relevant standards as part of their production process.
However, current testing approaches, which were developed for earlier generation optical fiber communications links, may not be suitable for the more advanced systems that are currently being developed and fielded. In one testing approach, transmitters are tested based on the quality of their immediate output. For example, the signal to noise ratio (SNR), bit error rate, eye diagram or other performance metric may be measured at the output of the transmitter. This approach ignores interactions between the transmitter and other components in the system (e.g., the optical fiber and the receiver) and, as a result, may be too simplistic to give an accurate assessment of the overall system performance achievable by the transmitter. For example, if the optical fiber communications link is designed so that the receiver compensates for distortions or other impairments introduced by the channel or the transmitter, a direct measurement of the performance at the transmitter output may not accurately reflect this subsequent compensation. In fact, in certain systems, it may be difficult to obtain an accurate assessment of the performance of the transmitter based only on its immediate output. Similarly, standards for transmitters used in more advanced systems may be based in part on their interaction with other components. For example, if the standard assumes a certain type of distortion mitigation in the receiver, the transmitter may be allowed a certain level of distortion that the receiver can compensate, but other types of distortion that the receiver cannot compensate should be limited. A transmitter test that cannot distinguish between the two types of distortion (an eye diagram, for example) will be an inadequate compliance test for this type of standard. Hence, a test of the transmitter alone may not be sufficient to determine whether the transmitter complies with the standard.
Another testing approach uses hardware reference components to emulate the overall optical fiber communications link. For example, there are a number of standards that relate to 10G fiber networks. 10G Ethernet over optical fiber is specified in IEEE Std 802.3ae-2002 Media Access Control (MAC) Parameters, Physical Layers, and Management Parameters for 10 Gb/s Operation (abbreviated herein as IEEE 802.3ae). The IEEE 802.3ae standard includes a Transmitter and Dispersion Penalty (TDP) measurement. The TDP measurement was developed as a transmitter compliance test to ensure that a compliant transmitter will communicate with a compliant receiver over a compliant channel with acceptable performance. The TDP measurement involves signal quality characteristics such as rise and fall times, eye pattern opening, jitter, and other distortion measurements. The intent is to insure that the signal received from a compliant transmitter can be detected to deliver a specified bit error rate.
The TDP measurement technique described in the 802.3ae standard involves a hardware test setup, including a hardware reference transmitter, the transmitter under test, a reference channel (i.e., actual length of fiber), and a hardware reference receiver. The test proceeds as follows. A reference link is established by connecting the reference transmitter to the reference receiver via the reference channel. The performance of the reference link is measured. A test link is established by connecting the transmitter under test to the reference receiver via the reference channel. The performance of this link is also measured. The performance of the reference link is compared with the performance of the test link to arrive at a value of TDP. The reference transmitter and reference receiver are high-quality instrument-grade devices. The test procedure involves several manual calibration and measurement steps. There are several disadvantages to this approach.
One disadvantage is the need for a hardware reference transmitter, a hardware reference channel, and a hardware reference receiver. The reference transmitter, channel, and receiver have tightly defined characteristics and are therefore very expensive. For example, the reference transmitter has tight specifications on rise and fall times, optical eye horizontal and vertical closure, jitter, and relative intensity noise (RIN). Furthermore, it is possible that different reference transmitters, channels, or reference receivers might yield different results due to parameter tolerances, environmental conditions (temperature, aging), and other differences. Therefore, the recommended measurement procedure involves a number of complex and sensitive calibration steps. The accuracy of the TDP measurement is sensitive to these time consuming and difficult calibration steps. Since these calibration steps all have tolerances, there may be significant variation in the results.
Another disadvantage is that the hardware approach is not easily extendible to other standards. Another standards committee is IEEE 802.3aq, which is developing a standard (10GBASE-LRM) for 10G Ethernet over multi-mode fiber over distances of up to 220 meters using electronic dispersion compensation (EDC). This standard is in a draft state, currently documented in IEEE Draft P802.3aq/D3.0, Draft amendment to: IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements, Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, Amendment: Physical Layer and Management Parameters for 10 Gb/s Operation, Type 10GBASE-LRM, referred to herein as IEEE 802.3aq/D3.0. The use of EDC in the receiver allows communication over longer distances and/or use of lower cost components. Some of the added waveform distortions can be corrected in the EDC receiver. However, this leads to some problems in defining and measuring a metric for transmitter qualification.
For example, TDP is not an appropriate transmitter compliance test for 10GBASE-LRM (EDC) systems because the reference receiver does not include equalization and does not accurately compensate for those distortions that a receiver using EDC can clean up. In general, there are certain signal impairments that can be corrected by an EDC receiver (such as most linear distortions), and there are other impairments that cannot normally be equalized (such as most nonlinear distortions). A suitable transmitter quality measure for an EDC system should measure the signal quality (or resulting bit error rate) achievable by a receiver with equalization. A reference receiver for such a system probably would include a decision feedback equalizer (DFE) as an EDC technique. However, a reference grade equalizer in the hardware receiver would be very difficult to make. It would have its own limitations, such as finite length and finite precision and tolerances, and these limitations could conceal the properties of the signal being measured. The resulting measurements may not accurately indicate correctable versus non-correctable impairments.
Another disadvantage with the use of TDP when applied to a transmitter intended for use on a link with an EDC receiver is the dependence of TDP on a measurement of Optical Modulation Amplitude (OMA). OMA is the difference in optical power for the nominal “1” and “0” levels of the optical signal. The measurement technique defined by 802.3ae is to capture samples of a test waveform using a sampling oscilloscope. The test waveform is the response of the transmitter to a square wave, which is a repetitive pattern of several consecutive 1's followed by an equal number of consecutive 0's. The mean optical power level of an optical 1 is measured over the center 20% of the time interval where the signal is high, and similarly for 0's when the signal is low. This is appropriate when the waveform is well-behaved in the sense that the levels during the measurement windows are constant and well-defined. However, a system that uses EDC in the receiver enables the use of lower cost transmitters that may result in distorted transmitted waveforms, for example as discussed further in U.S. patent application Ser. No. 11/033,457, “Use of Low-Speed Components in High-Speed Optical Fiber Transceivers,” filed Jan. 10, 2005. The response of the transmitter to a square wave input may differ dramatically from a well-behaved waveform due to bandlimiting, dispersion, and other distortions (such as ringing). This may make it difficult for an operator to accurately determine the true high and low levels. Since the value of TDP is sensitive to the measured OMA, this inaccuracy will directly affect the TDP measurement.
Thus, there is a need for improved transmitter testing techniques for communications links, including for example 10G optical fiber communications links where the receiver includes EDC.